1. Field of the Invention
The invention relates to microorganisms and processes for the fermentative preparation of L-cysteine, L-cystine, N-acetylserine or thiazolidine derivatives.
2. The Prior Art
It is well known to prepare many amino acids by fermentation. However, there has previously not been any economical process for the fermentative preparation of L-cysteine.
Thiazolidine derivatives and the corresponding hemithioketals are generally produced when cysteine is condensed with ketones or aldehydes. The chemical condensation of cysteine with different ketones or aldehydes, in particular with .alpha.-ketoacids, is known. The condensation takes place with the hemithioketal as the intermediate. The hemithioketal is produced by the nucleophilic attack of the free electron pair of the sulfur on the electron-deficient carbon atom of the aldehyde group or keto group. Ring closure with the elimination of water then leads to the corresponding thiazolidine derivative.
The formation of thiazolidine derivatives is shown in a general manner in the following reaction sequence. ##STR1##
In this reaction, R.sub.1 and R.sub.2 can denote any organic radicals.
The starting compounds are consequently in equilibrium with the thiazolidine derivative through the intermediate hemithioketal. For this reason, the hemithioketal is generally also present in aqueous solution in addition to the thiazolidine derivative.
According to the present invention, "thiazolidine derivative" is also understood as meaning an equilibrium of these substances with the corresponding intermediate hemithioketal.
It has not previously been reported that thiazolidines are direct metabolites of cells. All reports of the formation of thiazolidines by cells are based on the external addition, in an excess, of one of the starting compounds, usually L-cysteine. This cysteine is then converted into pyruvate by desulfhydration and deamination. The pyruvate is then reacted with the added cysteine. (Ronald C. Simpson et al., Biochimica et Biophysic Acta, 496 (1977), 12-19). Kredich et al. have reported, in J. of Biol. Chem. 248, 17: 6187-6196, that 2-methyl-2,4-thiazolidinedicarboxylic acid is formed in vitro when L-cysteine is subjected to an enzymic desulfhydration. These authors consider that it is extremely unlikely that this substance is formed in vivo.
It is known to use thiazolidines as racemic precursors for preparing L-cysteine by means of biotransformation (EP-A 0 101 052, Ok Hee Ryu et al., Biotechnology Letters 17 No. 3, 275-280 (March 1995)). When the racemate is employed for preparing L-cysteine, it has to be converted stereoselectively into L-cysteine using enzymes or whole cells. The remaining diastereomers have to then be racemized once again. For these reasons, this biotransformation has a high cost.
The chemical synthesis of thiazolidines from racemic cysteine and a corresponding ketone or aldehyde leads to four different diastereomers. Carrying out a chemical synthesis from enantiomerically pure L-cysteine is expensive. There are difficulties in subsequently isolating the L-cysteine. For this reason, a process for preparing thiazolidine diastereomers which possess the R configuration at the C4 atom suffers from the high costs of the starting compounds.